Stroboscope, instrument that provides intermittent illumination of a rotating or vibrating object in order to study the motion of the object or to determine its rotary speed or vibrationfrequency. A machine part, for example, may be made to appear to slow down or stop; the effect is achieved by producing illumination in very short, brilliant bursts that always occur when the moving part is in the same phase of its motion.

Early stroboscopic devices utilized either intermittent vision or interrupted light; in both cases a spinning or oscillating disk with a narrow radial slot either allowed the object to be viewed at regular intervals or permitted light to illuminate it at successive instants, thus exposing it at precisely the times it reached a given point in its motion.

The modern electronic stroboscope employs a gas-filled discharge lamp to produce very short, repetitive, brilliant flashes of light. Typically, a flash duration of about one microsecond (0.000001 second) and flashing rates ranging from 110 to 150,000 per minute are achieved. Using special techniques, flashing rates of more than 500,000 per minute have been obtained.

The brilliant short-duration flash produced by a stroboscope is admirably suited for photographing rapidly moving objects. Single flashes with durations of one millionth of a second can be used in such photography, while for ordinary photography, flash durations of one thousandth of a second are common.

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...rotated with the shaft being measured produces eddy currents that are proportional to angular speed. Electric-generator tachometers work by generating either an alternating or a direct current. The stroboscope, an instrument that illuminates rotating objects so that they appear to have stopped moving, can be used as a tachometer.